The proposed method simultaneously reconstructs activity and attenuation distribution of SPECT scans without usage of additional transmission scans. Moreover, in contrast to other approaches, it effectively prevents cross-talk artefacts by using a-priori atlas data and by labelling each organ with homogeneous attenuation values. The method generates a 3D-shape model of the patient and, in order to improve overall consistency between measured and estimated SPECT sinogram, modifications to the activity- and attenuation estimation are performed iteratively. Several reconstructions of patient and simulated SPECT data were investigated and reliable convergence behaviour as well as good agreement with reference images could be observed.

Temporal subtraction techniques using 2D image registration improve the detectability of interval changes from chest radiographs. Although such methods are well known for some time they are not widely used in radiologic practice. The reason are strong pose differences between these follow-up acquisitions with a time interval of months to years in between. Such strong perspective differences occur in a reasonable number of cases. They cannot be compensated by available image registration methods and thus mask interval changes to be undetectable. A method is proposed to estimate a 3D pose difference by the adaptation of a 3D rib cage model to both projections. The difference between both is then compensated for, thus producing a subtraction image with virtually no change in pose. No 3D image data is used. The accuracy of pose estimation is validated with chest phantom images under controlled geometric conditions.

Abstract: A pixelated X-ray semiconductor detector (=direct converter) is studied which contains an inhomogeneous electric field parallel to the depth axis caused by different concentrations of p- or n-doping. The X-ray energy deposition and charge movement within the detector is modeled in Monte-Carlo simulations which give access to astatistical analysis of electron drift times and current pulse widths for various degrees of static polarization. Integral charges induced on the pixel electrodes are evaluated and put to histograms of spectral detector responses and pulse height spectra (considering energy measurements before and after electronically pulse shaping, respectively). For n-doped semiconductors, the detector performance degrades due to pulse broadening. In contrast, a moderate p-doping can improve the detector performance by generating sharper electron pulses, as long as the detector is not limited by dynamical polarization.Conclusions: We performed Monte-Carlo simulations of energy deposition and charge movement within pixelated photon counting direct conversion detectors made of doped semiconductors of different acceptoror donor concentrations. Induced currents were statistically evaluated with the help of histograms of pulse width and total integral charge (represented before consideration of pulse shaper electronics in spectral responses, and after consideration of pulse shaper electronics in pulse height spectra, respectively). The electrical field close to anode pixels was identified as the main quantity defining the pulse characteristics. For n-doped semiconductors, a weaker electric field near the pixel anodes and longer total electron drift timesare seen, which results in broader pulse widths and a degradation of the spectral responses (by enhanced charge sharing) and pulse height spectra. In contrast, for p-doped semiconductors, a strengthenedelectric field near the anodes and an, on average, reduced total drift time is seen, which results in shorter pulse widths. For moderatep-doping concentrations n_A &#8804; 0.9n_max, an improvement of the spectral response (less charge sharing) and pulse height spectrumis seen. For p-doping concentrations close to the limit still allowing full depletion, n_A > 0.9n_max, the spectral response und pulseheight spectrum degrade again due to charges which experience long drift times when created in a region with weakened electric field near the cathode. We conclude that using moderately p-doped semiconductor material, n_A &#8776; 0.5n_max, improves detector performance aslong as the detector is not limited by dynamical polarization. In the latter case, it has to be noted that a p-doped semiconductor reduces the maximum count rate at which catastrophic dynamical polarization occurs.

In the context of dynamic contrast enhanced breast MR imaging we analyzed the effect of motion compensation by registration on the characterization of lesions. Two different registration techniques were applied: (1) rigid registration and (2) elastic registration based on the Navier-Lamé equation is applied. Interpreting voxels that exhibit a decline in image intensity after contrast injection (compared to the uncontrasted native image) as motion outliers, it can be shown that the rate of motion outliers can be largely reduced by both rigid and elastic registration. The performance of four lesion features including maximal signal enhancement ratio and variance of the signal enhancement ratio measured by area under the ROC curve as well as Cohen's κ value improved for elastic registration only, whereas features derived from rigidly registered images did not reach the performance level of either unregistered or elastically registered data.

C-arm based tomographic 3D imaging is applied in an increasing number of minimal invasive procedures. Due to the limited acquisition speed for a complete projection data set required for tomographic reconstruction, breathing motion is a potential source of artifacts. Intra-scan motion estimation and compensation is required. Here, a scheme for projection based local breathing motion estimation is combined with an anatomy adapted interpolation strategy and subsequent motion compensated filtered back projection. This approach is applied in animal experiments on a flat panel C-arm system delivering improved image quality in 3D liver tumor imaging.